The fire was reported at 9:39 p.m. Thursday off of the Silverado Trail in the Soda Canyon area of the county, fire officials said.
Evacuation orders were given for residents and businesses on Soda Canyon Road and Ridge, Loma Vista and Shady Oaks drives, according to Cal Fire.
Between 40 to 50 homes were initially threatened, but fire officials said crews were gaining the upper hand on the fire this morning. The blaze destroyed one outbuilding but has not destroyed any homes.
The Napa High School gymnasium was opened to evacuees and the American Red Cross responded to assist them, Napa County sheriff's deputies said.